1. Field of the Invention
This invention relates to bi-directional semiconductor devices, and more particularly, to a bi-directional transistor structure.
2. Description of Related Art
A MOS (metal-oxide semiconductor) transistor is called bi-directional if its source and drain can be interchanged for use, and unidirectional if they cannot. In the literature of this specification, the term "source/drain electrode" is used to refer to such an electrode of the MOS transistor that can be used either as a source or as a drain. In 0.25 .mu.m submicron ULSI (ultra-large-scale integration), the undesired effect of hot carrier injection (HCI) in bi-directional MOS transistor elements is about 10.sup.2 times worse than that of uni-directional devices. A semiconductor device with bi-directional transistor elements is therefore considerably inferior in performance than that with uni-directional ones. The HCI effect is unsymmetrical and this causes the forming of undesired inter-faces and the gathering of carriers near the drain junction in the bi-directional device. These in turn form charge traps that degrade the performance of the device.